I've found out the culprit, or culprits
1) I had enabled snapshots on the first volume. Snapshots on lvm have a huge impact on lvm volume write speeds (google lvm snapshots performance) I noted speed reductions of a 10th of the speed on volumes wo snapshots. See attachment.
2) I have VMs using VirtIO Red Hat drivers in Linux. these sometimes "hang" for 15-30mins on shutdown.

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I've found out the culprit, or culprits
1) I had enabled snapshots on the first volume. Snapshots on lvm have a huge impact on lvm volume write speeds (google lvm snapshots performance) I noted speed reductions of a 10th of the speed on volumes wo snapshots. See attachment.
2) I have VMs using VirtIO Red Hat drivers in Linux. these sometimes "hang" for 15-30mins on shutdown.

Good point!
Not sure why it needs to read or check snapshots in the boot sequence.